Thermoplastic materials are commonly used to manufacture various shaped articles which may be utilized in applications such as automotive parts, food containers, signs, packaging materials and the like. One such article is a profile. Profiles are defined by having a particular shape and by their process of manufacture known as profile extrusion. A common shape of a profile is tubing. Profiles are not film or sheeting, and thus the process for making profiles does not include the use of calendering or chill rolls. Profiles are also not prepared by injection molding processes. Profiles are fabricated by melt extrusion processes that begin by extruding a thermoplastic melt through an orifice of a die forming an extrudate capable of maintaining a desired shape. The extrudate is typically drawn into its final dimensions while maintaining the desired shape and then quenched in air or a water bath to set the shape, thereby producing a profile. In the formation of simple profiles, the extrudate preferably maintains shape without any structural assistance. With extremely complex shapes, support means are often used to assist in shape retention. In either case, the type of thermoplastic resins utilized and its melt strength during formation is critical.
Polymers typically used for making profiles include poly(vinyl chloride) (PVC), acrylic polymers, and polycarbonate. Each of these polymers suffers from one or more disadvantages. For example, PVC is undesirable from an environmental standpoint. Acrylic objects are brittle and shatter when dropped or struck against another object. Polycarbonate is too expensive for many applications.
Polyesters have also been used for making profiles, however, severe processing problems exist due to inadequate melt strength at typical profile processing temperatures of 480-550.degree. F. (250-290.degree. C.). Processing line speeds vary considerably depending on the shape of the profile. Typical speeds for simple shapes like a corner guard may be from about 50 to about 70 feet per minute. More complicated shapes may have process line speeds as low as 1 foot per minute, whereas extremely simple shapes with certain types of processing technology may run at speeds as high as 100 feet per minute. At the higher speeds, which obviously would be preferred by profile manufacturers, inadequate melt strength produces an extrudate that does not maintain its shape prior to quenching, and thus deformation occurs. To increase the melt strength of the polyester, processing temperatures and speeds are lowered. By decreasing speed, the economic attractiveness of using polyesters is also decreased. Thus, the profile extrusion processes are often operated at maximum speeds associated with the highest temperatures and minimal melt strengths for maintaining particular profile shapes. Any increase in speed or lowering of temperature may cause an increase in high shear viscosity in the die, which then may cause an undesirable phenomenon known as melt fracture.
Melt fracture is a flow instability phenomenon occurring during extrusion of thermoplastic polymers at the fabrication surface/polymer melt boundary. The occurrence of melt fracture produces severe surface irregularities in the extrudate as it emerges from the orifice. The naked eye detects this as a frosty appearance or matte finish as opposed to an extrudate without melt fracture that appears clear. Solutions for reducing melt fracture include streamlining the flow channel geometry, increasing the cross-sectional area of the orifice, increasing resin temperatures, reducing processing speeds, reducing the melt viscosity of the resin, and utilizing an external lubricant.
Thus, there exists a need in art to have polyester compositions which can be made into profiles by having improved processing characteristics during their extrusion to significantly reduce melt fracture while not sacrificing melt strength and speed. Accordingly, it is to the provision of such that the present invention is primarily directed.